Modeling and Analysis of Litz Wire Radio Frequency (RF) Coil in Inside-Out NMR Well Logging Sensor

Nuclear magnetic resonance (NMR) well logging is an important method of oil measurement, and the radio frequency (RF) coil is an important component for obtaining accurate parameters of rock formation. The use of Litz wire as RF coil can help minimize the resistance and increase the signal-to-noise ratio (SNR) of NMR signals. However, the electromagnetic field computing and performance evaluation of Litz wire are complicated and difficult. In this study, we proposed an effective model for analyzing the Litz wire in inside-out NMR logging sensor. Considering the electromagnetic parameters of the coil itself, the frequency response of excitation, and tuning and matching circuit, we proposed a new and accurate model for SNR analysis. We then introduced a homogenization-based finite-element method (FEM) to calculate the Litz wire around a ferrite core, and the electromagnetic parameters of Litz wire with different structures were obtained accurately and quickly. We compared the Litz wire and enameled wire through our proposed model. Results show that the SNR of Litz wire is remarkably higher than that of enameled wire, and the model analysis results are consistent with the measured results. Suggestions for further reducing the ac resistance of RF coil composed of Litz wire are provided.